This invention relates generally to production of magnetic material, and more particularly to improved processes for the production of magnets characterized by superior magnetic properties.
It is known that magnets made of an alloy of iron, neodymium and boron are characterized as having remarkably high coercitivity as well as other improved magnetic properties. The particular alloy is based on the Fe-Nd-B family of rare earth transition metals materials, otherwise designated "FeNdB." These materials are manufactured using rapid solidification technology.
Such FeNdB magnets combine the highest known magnetic energy product with the high polarization coercitivity, jH.sub.c. These parameters are the most important to characterize permanent magnet performance. In addition to this, Nd has a considerable price advantage, and fewer supply restrictions than samarium and/or cobalt, the latter being the main components of the established RE permanent magnets.
There are two basic technological processes used to prepare FeNdB magnets. One of these is a traditional PM approach consisting of alloy preparation, pre-milling, milling, control and adjustment of the composition, particle alignment and pressing, sintering and heat treatment. An alternate method of preparing FeNdB magnets is by using rapidly solidified (RS) materials. Larger coercive forces can be attained by melt-spinning of rare earth iron alloys due to the formation of a metastable phase and a very fine microstructure compared to a classically obtained powder. The most simple approach of manufacturing melt-spun FeNdB ribbons preserving the characteristics gained by RS, is to compact them and glue the ribbon fragments together. The RS FeNdB alloy ribbon is crushed before blending with the glue (an epoxy resin). The reported maximum energy product (BH).sub.max is 8 MGOe(63KJ/m.sup.3) [3]. The theoretical maximum energy product for FeNdB materials is 64 MGOe(500 KJ/m.sup.3). Hot pressing of crushed ribbons increases the maximum energy product to 13-15 MGOe (102-118 KJ/m.sup.3). Next step deformation by die upsetting of hot pressed RS materials results in an anisotropic magnet with (BH).sub.max of 20-40 MGOe (158-316 KJ/m.sup.3). Milling of the RS melt-spun ribbons results in smaller particle size. The coercive force of ground powders decreases with decreasing particle size. This reduces coercitivity of the permanent magnets. [4].
All the above mentioned processes of consolidating RS materials involve crushing of RS powders. Besides melt-spun powders there are other methods to manufacture Rs FeNdB powders.
The high reactivity of the rare earths and their alloys, and the critical dependence of the magnetic properties in the chemical composition, require effective suppression of contamination during the powder metallurgical processing. In order to prevent oxidation of the melt-spun FeNdB alloy, an inert gas atmosphere is required in each step of powder milling or consolidation.